Java Lambda Expression: incompatible types: bad return type in lambda expression

Question

I have the following list of jobs, each element is a list containing profit and difficulty:

List<List<Integer>> jobs = new ArrayList<>();
for (int i = 0; i < difficulty.length; i++) {
    List<Integer> job = new ArrayList<Integer>();
    job.add(profit[i]);
    job.add(difficulty[i]);
    jobs.add(job);
}

Now I want to sort the jobs based on their profit (first element of each job) like below:

jobs.sort((j1, j2) -> j1.get(0) > j2.get(0));

But got the following error:

error: incompatible types: bad return type in lambda expression

What did I do wrong and how do I fix this? Thanks!

Answer 1

Your comparator is invalid as it returns a boolean whereas the expected return type is an int.

A simple solution would be:

jobs.sort(Comparator.comparing(e -> e.get(0)));

or:

jobs.sort((j1, j2) -> Integer.compare(j1.get(0), j2.get(0)));

Answer 2

The problem is in following line

jobs.sort((j1, j2) -> j1.get(0) > j2.get(0));

is that you don't follow signature of sort method:

default void sort(Comparator<? super E> c)

When you implement comparator as lambda, you should pass lambda that returns int, not boolean.

in your case it should be smth like this:

jobs.sort((j1, j2) -> compare two lists somehow to return int);

or use snippet that @Aominè provided